Separation membranes may be made from various inorganic or organic materials, including ceramics, metals and polymers. Many membranes are asymmetric in structure, and incorporate a finely porous surface layer or skin and a much more open microporous substrate. The finely porous surface layer performs the separation; the microporous substrate provides mechanical strength. Such membranes may be integrally asymmetric, for example polymer membranes made by the Loeb-Sourirajan process, and used for reverse osmosis and gas separation applications, or may be composite structures in which the support and selective layers are formed in separate operations, and made from different materials, for example ceramic composites used for certain ultrafiltration and nanofiltration applications.
Composite membranes that include both inorganic and organic layers have also been proposed for some types of membrane separation application.
U.S. Pat. No. 3,544,358, to Universal Water Corp., describes a reverse osmosis membrane comprising a cellulosic derivative, such as cellulose acetate, applied as a selective layer onto a porous ceramic support.
U.S. Pat. No. 6,093,325, to Bechtel BWXT Idaho, describes a membrane for diffusion dialysis, pervaporation or reverse osmosis separations. The membrane comprises a thin polyphosphazene layer cast onto a porous inorganic or polymer support.
PCT Patent Application WO03/072232 A1, to Creavis Gesellschaft, describes a membrane comprising a ceramic support and an organic polymer selective layer.
U.S. Pat. No. 5,066,398, to Societe Des Ceramiques Techniques, describes a pervaporation membrane comprising a porous inorganic support coated with a continuous separating layer of a polyphosphazene. Penetration of phosphazene into the pores of the support is limited to a depth of less than 5 times the pore diameter.
U.S. Pat. No. 5,266,207, to Techsep, describes a nanofiltration membrane comprising a porous inorganic support coated with a selective layer of an elastomeric polyphosphazene.
U.S. Pat. No. 4,861,480, to Commissariat a L'Energie Atomique, describes a reverse osmosis membrane comprising an inorganic porous support coated with a dense, semipermeable layer of poly(vinylidene fluoride) [PVDF]. An ethylenically unsaturated monomer is then grafted onto the PVDF layer, and the resulting membrane is functionalized to give the layer its separation capabilities.
U.S. Pat. Nos. 5,141,649 and 5,171,449, to Texaco, describe pervaporation membranes comprising a non-porous, cross-linked polyvinyl alcohol selective layer formed in situ on a porous ceramic support.
U.S. Pat. No. 6,440,309, to Yoram Cohen, describes a pervaporation membrane comprising a porous ceramic support onto which is graft-polymerized a vinyl lower alkoxysilane.
U.S. Pat. No. 4,874,516, to NGK Insulators, describes a “semi-ultrafiltration” membrane comprising a porous ceramic support coated with a membrane-forming fluorocarbon resin which provides the selective layer and partially permeates the pores of the support.
U.S. Pat. No. 5,342,521, to Commissariat a L'Energie Atomique, describes reverse osmosis or nanofiltration membranes having a porous, inorganic support, an intermediate mesoporous metal oxide layer, and a polymeric selective layer.
In all of the above cases, the ceramic substrate provides mechanical strength and the polymer coating layer is the selective layer that provides and governs the separation or rejection properties.
Many different types of polymers may be used for the selective layer of a membrane. Polyamide-polyether block copolymers have been reported to be useful as selective layers in polymeric gas separation and ultrafiltration membranes, as in U.S. Pat. No. 4,963,165; German patent number DE 4237604; an article by K. Ebert et al., “Solvent resistant nanofiltration membranes in edible oil processing,” (Membrane Technology, No. 107, p. 5-8, 1999); and an article by S. Nunes et al., “Dense hydrophilic composite membranes for ultrafiltration,” (J. Membrane Science, Vol. 106, p. 49-56, 1995).
Ultrafiltration is a membrane separation process that uses finely porous membranes to separate water and microsolutes from macromolecules and colloids. Ultrafiltration membranes operate by permeating water and small solutes and rejecting the larger dissolved or suspended materials. A driving force for water permeation is provided by applying an elevated pressure to the feed liquid or a reduced pressure on the permeate side, or both. At least at low pressure, the water flux through the membrane increases with increasing pressure difference across the membrane.
Ultrafiltration membranes are very susceptible to fouling. Fouling occurs when contaminants such as charged solutes, oils, bacteria, colloidal materials of various types, and suspended particulates become trapped on the surface or in the pores of the membrane. In addition to clogging pores, the accreting material forms a thickening gel layer on the membrane surface that presents an increasing resistance to water permeation. Thus, fouling impairs the membrane performance by progressively diminishing the transmembrane flux. For a short time, the increasing resistance presented by the fouling layer can be overcome by increasing the pressure driving force.
To help control membrane fouling, ultrafiltration systems may be designed to include one or more pretreatment steps upstream of the ultrafiltration units. These treatments typically include gravity separation and/or coarser filtration steps to remove potential foulants. In addition, frequent mechanical and/or chemical cleaning procedures are required. Although backflushing and chemical cleaning can remove the surface gel layer reasonably well, they are less successful in removing material trapped inside the membrane pores.
Despite use of the above procedures and operating protocols, fouling continues to be a significant problem in at least some applications and reduces the efficiency of many ultrafiltration processes.
There remains a need for intrinsically less fouling ultrafiltration membranes. If such a need could be filled, wider applications of ultrafiltration and nanofiltration treatment, such as to industrial and oily wastewaters of many types, or for military or naval use, would be possible.